Regio- and chemoselective oxidation of the bis(bidentate) phosphine cis,trans,cis-1,2,3,4-tetrakis(diphenylphosphino)cyclobutane via cobalt(II) mediated dioxygen activation

The bis(bidentate) phosphine cis,trans,cis-1,2,3,4-tetrakis(diphenylphosphino)cyclobutane (dppcb) has been regioselectively oxidized leading to novel, hemilabile ligands. [Co(2)Cl(4)(dppcb)] (1a) is transformed via cobalt(II) mediated dioxygen activation into [Co(2)Cl(4)(2,3-trans-dppcbO(2))] (2a) in excellent yield, where 2,3-trans-dppcbO(2) is cis,trans,cis-2,3-bis(diphenylphosphinoyl)-1,4-bis(diphenylphosphino)cyclobutane. By contrast, the in situ presence of dioxygen during the synthesis of Co(2)Br(4)(dppcb)] (1b) produces both [Co(2)Br(4)(2,3-trans-dppcbO(2))] (2b) and [Co(2)Br(4)(1,3-trans-dppcbO(2))] (3), where 1,3-trans-dppcbO(2) is cis,trans,cis-1,3-bis(diphenylphosphinoyl)-2,4-bis(diphenylphosphino)cyclobutane. The new compounds 2a, 2b and 3 have been obtained as pure, crystalline solids and all three X-ray structure analyses have been performed showing folded cyclobutane rings. Interestingly, the corresponding reaction using [Co(2)I(4)(dppcb)] (1c) proceeds chemoselectively. Thus, [Co(2)I(4)(dppcbO(3))] (4), where dppcbO(3) is cis,trans,cis-1,2,3-tris(diphenylphosphinoyl)-4-diphenylphosphinocyclobutane, is formed in excellent yield and also fully characterized by an X-ray structure analysis showing two different conformations of 4. However, [Co(2)(NO(3))(4)(dppcb)] (1d) shows no dioxygen activation at all. Therefore, in order to reveal the mechanism of this oxidation [Co(2)I(4)(DMF)(2)(dppcb)] (5) has been prepared and its X-ray structure is presented. The synthesis of [Co(2)I(4)(PMe(2)Ph)(2)(dppcb)] (6) proves that this is a common reaction pathway. Furthermore, because the product distribution of the oxidation strongly depends on the kind of halides present, the whole series Co(2)X(4)(dppcbO(4))] (X = Cl, 7a; Br, 7b; I, 7c) has been prepared, where dppcbO(4) is cis,trans,cis-1,2,3,4-tetrakis(diphenylphosphinoyl)cyclobutane, and all three X-ray structures are given, also showing folded cyclobutane rings. It seems likely that coordination of dppcb to cobalt(II) is essential to form the regio- and chemoselectively oxygenated molecules.